2.(Crystallog.) The quality possessed by many crystallized substances of splitting readily in one or
more definite directions, in which the cohesive attraction is a minimum, affording more or less smooth
surfaces; the direction of the dividing plane; a fragment obtained by cleaving, as of a diamond. See Parting.

3.(Geol.) Division into laminæ, like slate, with the lamination not necessarily parallel to the plane of
deposition;  usually produced by pressure.

Basal cleavage

, cleavage parallel to the base of a crystal, or to the plane of the lateral axes. 